The present invention relates to a programmable logic array employing superconducting devices, especially Josephson devices, and also to read-only memories for use in the logic array.
Since a Josephson device is switched at high speed and dissipates low power, it has a significant potential for application to digital systems such as computer. A prir art arrangement has taken an expedient wherein a Josephson LSI having a complicated logic is constructed by combining individual AND circuits and OR circuits. This method, however, has the disadvantage that logic amendments due to a design alteration and an erroneous design cannot be coped with quickly. Particularly a system employing Josephson devices operates under only the environment of extremely low temperature (about 4.degree. K.), exhibits as small a signal amplitude voltage as 2.5 mV and affords fast circuit operations on the order of several ps. It is accordingly difficult to follow up logic operations with a probe or the like. Therefore, it is impossible to repeat a large number of small amendments, and the logic amendments must be met by performing a small number of large amendments on the scale of the whole LSI. Consequently, the system employing the Josephson devices needs to quickly perform the amendments on the scale of the whole LSI, but this is equivalent to re-designing the LSI. The LSI in the prior art, in which the logic is organized by the combination of the AND circuits and OR circuits, requires a high design cost and a long design time. Therefore, it has the disadvantages that it is expensive and that a long time is required for finishing a system.
U.S. Pat. No. 4,360,898 discloses a programable logic array comprising in combination two logic arrays in each of which Josephson devices are disposed at selected ones of the intersection points between a plurality of superconducting loops and a plurality of superconducting wiring leads or loops in a direction in which they intersect the superconducting loops. The known programable logic array is driven by D.C. power source means. Therefore, after the operations of each cycle have been performed, the Josephson devices for coupling input signals need to be reset by any method. Another disadvantage is that, since direct currents flow to the loops in succession, the access speed is low.